High spatial resolution (10-50 μm) analysis of Sr isotopes in rock-forming apatite by LA-MC-ICP-MS

In situ Sr isotopes analysis of apatite by LA-(MC)-ICP-MS is challenged by the difficulty to monitor and correct isobaric interferences from atomic and polyatomic ions. We present a new routine procedure for analysing rock-forming apatites with a Thermo Scientific Neptune XT MC-ICP-MS coupled with a Teledyne Cetac Analyte Excite+ 193 nm laser ablation system. Five apatite standards that cover a large range of REE/Sr ratios were selected, and their ⁸⁷Sr/⁸⁶Sr ratios were measured in solution after dissolution and purification of Sr [Durango: 0.706321(5); Madagascar: 0.711814(5); Slyudyanka; 0.707705(4); Sumé: 0.707247(4); and Ipirá: 0.710487(4)]. The optimisation of both instrument setup and data reduction schemes was achieved through repeated measurements of calibration solutions and of apatite standards at four different rectangular-shaped laser ablation beam sizes (50 × 50, 25 × 25, 13 × 13 and 10 × 10 μm). Two complementary methods were developed for data reduction: Method 1, which corrects measured intensities for gas blank and instrumental mass bias only; and Method 2, which additionally corrects for isobaric interferences of ⁸⁷Rb⁺, ^{166, 168 and 170}Er⁺⁺, ^{170, 172, 174 and 176}Yb⁺⁺, ⁴⁰Ca⁴⁴Ca⁺, ⁴⁰Ca⁴⁶Ca⁺, ⁴⁴Ca⁴³Ca⁺ and ⁴⁰Ca⁴⁸Ca⁺. A precision of ca. 100 ppm (2 s.e.) can be achieved on the ⁸⁷Sr/⁸⁶Sr ratio with a 50 μm laser ablation beam when using Method 2, and it remains better than 3000 ppm at 10 μm with Method 1. Method 1 gives precise and accurate ⁸⁷Sr/⁸⁶Sr ratios when ¹⁷³Yb⁺⁺ is below the global limit of detection (with LOD_global = 3 s.d. of the means of all gas blanks measurements). When ¹⁷³Yb⁺⁺ is above the LOD_global, Method 2 should be preferred as it provides more accurate ⁸⁷Sr/⁸⁶Sr ratios. Overall, this study offers a robust and reliable approach for LA-MC-ICP-MS analysis of Sr isotopes in rock-forming apatite at a high spatial resolution (i.e. down to 10 μm), overcoming previous limitations associated with instrumental set up and data reduction.